The present invention relates to a method for forming a semiconductor monocrystalline layer on an insulating layer formed on one of the main planes of a monocrystalline semiconductor substrate. More specifically, the invention relates to such a method in which the semiconductor monocrystalline layer is formed by melting a polycrystalline or amorphous semiconductor layer formed on the insulating layer with a CW laser to thereby monocrystallize the polycrystalline or amorphous semiconductor layer with the monocrystalline semiconductor material forming the substrate as a seed.
FIG. 1 shows an example of the conventional method of forming a monocrystalline semiconductor layer on an insulating layer. In FIG. 1, on a main surface of a monocrystalline silicone substrate 11, specifically, a surface which corresponds to a 100 crystal face thereof, a relatively thick silicon dioxide layer 12 is formed, on which a polycrystalline silicon layer 13 is grown by chemical vapor deposition (DVD). The polycrystalline silicon layer 13 is irradiated with a laser beam 15 to melt it. It is then allowed to solidify, thus monocrystallizing along the main plane of the substrate 11 to thereby form a monocrystalline silicon layer 14.
The polycrystalline silicon layer 13 is formed on the substrate 11 through steps shown in FIGS. 2a to 2d. As shown in FIG. 2a, the silicon substrate 11 having a main surface corresponding to a {100} crystal face of silicon is exposed to an oxidizing atmosphere at 950.degree. C. to form a thermal oxide layer 21 having a thickness of 500 .ANG. on the surface of the substrate. Then a silicon nitride layer 22 about 1000 .ANG. thick is formed thereon by CVD. Thereafter, portions of the silicon nitride layer 22 are removed by photolithography, as shown in FIG. 2b.
Further, by using the silicon nitride layer 22 as a mask, portions of the oxide layer 21 are removed and the silicon substrate 11 is etched to a depth of 5000 .ANG.. Then, the silicon substrate 11 is exposed in an oxidizing atmosphere at 950.degree. C. to form an oxide layer 12 of silicon dioxide having a thickness of about 1 .mu.m on the etched portion of the substrate as shown in FIG. 2c. Then the silicon nitride layer 22 and remaining portions of the oxide layer 21 on the silicon substrate 11 are removed, and thereafter a polycrystalline silicon layer 13 about 7000 .ANG. thick is grown by CVD. Thus, the polycrystalline silicon layer 13 is formed on the thick oxide layer 12 as shown in FIG. 2d, which is in direct contact with the substrate 11 through a region 23, hereinafter referred to as a window 23.
The polycrystalline silicon layer 13 on the window 23 is irradiated with a laser beam 15 as shown in FIG. 1 to melt the polycrystalline silicon layer 13 to a depth sufficient to reach the surface of the substrate. Afterwards, monocrystallization of the polycrystalline silicon layer 13 is achieved through solidification of the melted crystalline silicon layer 13. The laser beam 15 is moved relative to the polycrystalline silicon layer 13 in a direction indicated by an arrow X in FIG. 1 so that a melting and subsequent solidification of the silicon layer progresses in that direction to thereby realize continuous epitaxial growth of the monocrystalline layer, even on the oxide layer 12.
In this conventional method, however, the laser power necessary to melt the portion of the polycrystalline silicon layer 13 on the window 23 is different from that for the portion thereof on the oxide layer 12 because of a difference in thermal conductivity between the substrate and the oxide layer. That is, if the portion of the polycrystalline silicon layer on the oxide layer were irradiated with the same power of laser light necessary to melt the portion on the window, the former portion would be heated excessively, resulting in a tendency for the polycrystalline silicon layer to peel.
In order to resolve this problem, it has been proposed to control the laser power by providing separate antireflection layers on the surfaces of the portions of the polycrystalline silicon layer on the windows and the oxide layers. In this case, however, since the lateral heat distribution inside the polycrystalline silicon layer on the oxide layer is not controlled suitably during melting, the width of the epitaxially grown monocrystalline material at the window portion may not be constant, and no growth of the material may take place on the oxide layer. Also, due to the uncontrolled lateral heat distribution, even a small fluctuation of laser power distribution may cause discontinuities in the epitaxial growth of the monocrystalline material. That is, the power distribution of a laser beam is usually a normal or substantially normal distribution as shown in FIG. 3a, and when the melting and monocrystallization of the polycrystalline silicon layer are performed with such a laser beam, the temperature distribution in the polycrystalline silicon layer will be similar to that shown in FIG. 3a. Therefore, with the movement of the laser beam 31, the solid-liquid interface 32 may be moved as shown in FIG. 3b. That is, the direction 33 along which monocrystallization proceeds changes gradually towards a center line with the movement of the light spot, and thus a crystal grain boundary may form therealong, making uniform monocrystallization impossible.